1. Field of the Invention
This invention relates to a method for the production of acrylic acid, which method is characterized by preventing the bottoms of an azeotropic dehydration column from substantially containing a dehydrating solvent and water by causing the column top composition of the azeotropic dehydration column to have an acrylic acid concentration in the range of 0.06-0.80 wt. %.
2. Description of the Related Art
Acrylic acid is used in coating material, processed fibers, leather, and building material besides being utilized as a copolymer for acrylic fibers or as an adhesive agent in the form of emulsion. These applications have been placing a growing demand on the acrylic acid. Thus, the desirability of developing a process which allows use of an inexpensive raw material, permits expansion of production, and entails environmental pollution only sparingly has been finding popular recognition. The acrylic acid is generally produced by the reaction of catalytic gas phase oxidation of propylene, for example. The reaction of catalytic gas phase oxidation which yields acrylic acid as a target product, however, produces acrolein, aldehydes such as formaldehyde, compounds such as acetic acid which have lower boiling points than acrylic acid, and compounds such as acrylic acid dimer which have higher boiling points than acrylic acid as by-products thereof. This reaction, therefore, requires such steps as absorbing an acrylic acid-containing gas into an absorbent, removing contained aldehydes from the absorbent, depriving the absorbent of water, and then further separating low boiling substances and high boiling substances from the residue contained acrylic acid.
The acrylic acid itself, however, is chemically analogous with acetic acid particularly among other by-products mentioned above. These two compounds are not easily separated from each other because they possess similar physical and chemical properties such as vapor-liquid equilibrium. Thus, numerous suggestions have been made concerning a method for efficiently dehydrating and distilling an azeotropic agent from the acrylic acid-containing aqueous solution produced by the absorption of acrylic acid and further separating acetic acid therefrom by distillation.
The official gazette of JP-B-63-10,691, for example, discloses a method for producing purified acrylic acid by absorbing the gas formed by the catalytic gas phase oxidation of propylene and/or acrolein with molecular oxygen, supplying the resultant acrylic acid-containing aqueous solution to a dehydrating distillation column, then distilling this aqueous solution in the presence of an azeotropic solvent capable of forming an azeotropic mixture of water and acetic acid thereby expelling substantially the whole of the water and part of the acetic acid contained in the acrylic acid-containing aqueous solution, subsequently introducing the acrylic acid still retaining acetic acid therein as the bottoms of the column to an acetic acid separation column, distilling the persisting acetic acid in conjunction with the azeotropic solvent through the top of the acetic acid separation column, and meanwhile recovering the purified acrylic acid through the bottom of the column. The invention published in the official gazette mentioned above incidentally has been produced in view of the following problems. For example, the acetic acid cannot be effectively removed under the condition allowing no presence of the azeotropic solvent in the bottom part of the azeotropic dehydration column. An attempt to remove the acetic acid under the condition allowing no presence of an azeotropic solvent brings fatal defects such as suffering the acrylic acid to polymerize in the region from the middle part through the top of the distillation column and increasing the pressure drop of the column. For this reason, the practice of utilizing two columns, i.e. an azeotropic dehydration column and an acetic acid separation column, for the operation, using an azeotropic solvent in both the azeotropic dehydration column and the acetic acid separation column, expelling by distillation substantially the whole of water and part of acetic acid in the aqueous acrylic acid solution together with the azeotropic solvent mentioned above through the top of the azeotropic dehydration column, and subsequently subjecting in the acetic acid separation column the acetic acid and acrylic acid extracted through the bottom of the dehydration column to purification by the use of the azeotropic solvent mentioned above has been in vogue.
The official gazette of U.S. Pat. No. 5,910,607 discloses a method for the production of purified acrylic acid, which method comprises keeping the concentration of an azeotropic solvent in the range of 5-30 wt. % and the concentration of water not more than 0.5 wt. % in the bottoms of an azeotropic dehydration column, utilizing the azeotropic dehydration column and an acetic acid separation column for the operation, and setting the theoretical plate numbers of the columns each in the range of 5-20. Since acrylic acid is an unusually easily polymerizing substance, the distillation column after continuous operation tends to form a detestable polymer in the neighborhood of the bottom of the azeotropic dehydration column and in the neighborhood of the top of the acetic acid separation column. The invention published in this official gazette has been produced in view of this defective point. Specifically, by setting the concentration of the azeotropic solvent in the range of 5-30 wt. % and the concentration of water not more than 0.5 wt. %, preferably in the range of 0.3-0.05 wt. % in the bottoms of the azeotropic dehydration column, it is made possible to prevent the detestable polymerization of acrylic acid not only in the azeotropic dehydration column but also in the acetic acid separation column and allow the distillation column a lasting continuous operation. As a result, the method succeeds in removing inside the azeotropic dehydration column substantially the whole of water and part of acetic acid into the distillate emanating through the top of the column and obtaining an acrylic acid solution containing the acetic acid in the bottoms of the column and the azeotropic solvent having a concentration in the range of 5-30 wt. %. Particularly, the concentration of the azeotropic solvent in the bottoms of the azeotropic dehydration column is preferred to be as high as permissible from the viewpoint of preventing acrylic acid from polymerization. If the concentration is unduly high, however, the excess will be at an economic disadvantage in increasing the energy necessary for the separation of the azeotropic solvent in the acetic acid separation column. Further, in the acetic acid separation column, the concentration of the azeotropic solvent is preferred to be advantageous for the separation of acetic acid. Thus, the concentration of the azeotropic solvent has been limited to the range specified above. Incidentally, the invention of this official gazette sets the lower limit of the concentration of water in the bottoms of the azeotropic dehydration column at 0.05 wt. %. The reason for this lower limit is that if this concentration falls short of this lower limit, the shortage will be at a disadvantage in jeopardizing the prevention of polymerization by heightening the temperature of the bottom of the azeotropic dehydration column and further requiring the amount of the azeotropic solvent to be refluxed in the azeotropic dehydration column and, because of the consequent increase in the amount of acetic acid entrained in the distillate from the top of the column, inevitably inducing a decrease in the yield of the acetic acid to be recovered from the distillate of the acetic acid separation column.
The two-column type method of purification which comprises adding an azeotropic solvent to an acrylic acid-containing aqueous solution and subjecting this aqueous solution to azeotropic dehydration, then guiding the bottoms of the relevant azeotropic dehydration column to an acetic acid separation column, and purifying them therein through separation from acetic acid for the purpose of production of acrylic acid, however, requires the azeotropic solvent contained in the bottoms of the azeotropic dehydration column to be expelled by distillation in an acetic acid separation column operated at the subsequent step and necessitating energy for the separation and possibly entails difficulty in thoroughly removing the azeotropic solvent from the produced acrylic acid.
The unit cost of production can be lowered by rendering it possible to decrease the loss of the azeotropic solvent through the distillation column and the amount of the solvent to be used. Some of the substances which are discharged from the various distillation columns are reclaimable. When these effluents are simply discarded, the yield is lowered to the extent of rendering the production economically unfavorable and the discharged substances form a cause for environmental pollution.
Such being the case, the desirability of developing a method for the production of acrylic acid which operates satisfactorily with the existing equipment, allows the largest saving in the energy for purification, and prevents the reactants in the distillation columns from polymerization has been finding popular approval in the industry concerning the production of acrylic acid.
The present inventor, after making an elaborate study regarding the conditions for distillation to be used in an azeotropic dehydration column when an acrylic acid-containing aqueous solution is subjected to separation and purification by the use of at least two columns, i.e. the azeotropic dehydration column and an acetic acid separation column, has found that even when substantially the whole amount of an azeotropic solvent in the acrylic acid-containing aqueous solution is expelled by distillation from an azeotropic dehydration column into the distillate from the top of the column, the azeotropic dehydration column can be continuously operated for a long time without entailing polymerization of acrylic acid and that since the bottoms of the column do no longer contain the azeotropic solvent, the acetic acid separation column operated in the subsequent step obviates the necessity for separating the azeotropic solvent and excels in the efficiency of production and avoids inducing polymerization of acrylic acid. This invention has been perfected as a result. To be specific, the tasks imposed on this invention on account of the problems of the prior art mentioned above are accomplished by the following concept.
A method for the production of acrylic acid, comprising a step of subjecting propylene and/or acrolein to a reaction of catalytic gas phase oxidation, a step of absorbing the acrylic acid contained in the gas formed by the reaction of catalytic gas phase oxidation with an absorbent, and a step of dehydrating the acrylic acid-containing aqueous solution obtained by the step of absorption mentioned above, and characterized by the step of dehydration mentioned above adding and an azeotropic solvent capable of forming an azeotropic mixture with acetic acid and water and distilling the resultant mixture and adjusting the acrylic acid concentration at the top of the azeotropic dehydration column in the range of 0.06-0.80 wt. % thereby preventing the bottoms of the azeotropic dehydration column from substantially containing the azeotropic solvent and water.
In the method for the production of acrylic acid by the use of an azeotropic dehydration column and an acetic acid separation column, this invention can prevent acrylic acid from polymerization in the azeotropic dehydration column and the acetic acid separation column by merely adjusting the acetic acid concentration in the range of 0.06-0.80 wt. % with the tower top composition of the azeotropic dehydration column as a marker. Further, since acrylic acid and acetic acid are separated in the acetic acid separation column in the absence of the azeotropic solvent, the energy necessary for the distillation of the azeotropic solvent can be decreased Further, since the reactants in the acetic acid separation column contain an azeotropic solvent, the condenser attached to the column does not need to use chilled water as the cooling water therefore.
Further, this invention allows effective utilization of acrylic acid because it circulates partly or wholly the column top water phase and the column top distillate emanating from each of the azeotropic dehydration column and the acetic acid separation column to the acrylic acid absorption column and the azeotropic dehydration column and put them to use therein. It further allows the efficiency of the separation performed in the azeotropic dehydration column to be exalted by setting the site for feeding the column top distillate from the acetic acid separation column at a specific position in the azeotropic dehydration column.